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Image Source: Schiess

Image Source: Schiess

Hydrostatics—The Solution For High Precision In XXL Machine Tools

Does your customer have high requirements with regards to workpiece accuracy? One adjusting screw to guarantee a high accuracy is hydrostatic. Especially if you produce lot size 1 or large and complex workpieces, you have to avoid inaccuracy has to be avoided in the finished part, because if these parts are wasted, so is a lot of time and money. By Schiess GmbH

But what are the functions, advantages and disadvantages? References are also important for customers. Here you’ll find some answers to these questions.

Function/Explanation

Hydrostatic: An explanation | Image Source: Schiess

The contact surfaces of a guide or bearing are separated through a permanent oil film during the usage. The oil quantity is delivered from outside the machine. It is needed to generate and sustain the oil film. Therefore the height of the oil film is independent from the sliding velocity (one difference to hydrodynamic guides and bearings). How does it work? One side is equipped with pockets inside, e .g. the moveable gantry at our VertiMaster VMG. The oil is pushed into the pockets through supply lines. The distance between the pocket surface and the guide surface is named as the gap height h. The starting height h0 is about 10 µm to 60 µm (depends on the design and calculation of the pockets and the oil quantity). The pressure distribution is shown in the figure.

The pressure in the pocket pT is constant over the pocket and falls down to zero at the end of the pocket surface. If the oil aggregates are working, there will be a permanent oil film between the pocket surface and guide surface.

Advantages and disadvantages

The usage of hydrostatic guides and bearings has a lot of advantages:

  • Wear-free, therefore lower maintenance costs, higher machine availability and life cycle time
  • No change of the parameters of guides and bearings during the life cycle
  • High positioning accuracy
  • No start-up friction (pure fluid friction)
  • No stick-slip-effect
  • Excellent damping

The challenge—you have no advantages without disadvantages:

  • High manufacturing costs and challenging tasks to manufacture
  • High costs in case of additional aggregates

High Accuracy

In the opinion of Schiess GmbH, a German machine tool builder with over 160 years of experience, the usage of hydrostatic is a very important feature and advantages of our machines. They build up XL machines, aerospace machines and XXL machines (faceplate diameter till 12,000 mm, table length till 60,000 mm and longer, passage height till 8,000 mm). They offer a high product quality thanks to high manufacturing depth and quality (e.g. complete assembly in-house, air conditioned measuring laboratory, 100 percent measurement of the finished components) and high process quality thanks to extensive studies and support (e.g. preparation of stability maps as the basis for technology proposals (production optimisation).

Furthermore, there are some solutions to guarantee high precision manufacture without hydrostatics. Here you will see two of them.

Solution 1: Accuracy through process design

A machine tool manufacturer supports its customers during the selection of the right tools and process strategies. The reason: they don’t believe that they know everything better. But if they know the ideas and requirments of their customers from the beginning, they are better equipped to help at a later stage. Our application technology provide the complete process from the first ideas till the successful implementation in our instance.

Solution 2: Accuracy through design and usage of hydrostatic guides and bearings

Schiess XXL machines are equipped with hydrostatic guides in all linear axes. They are wear-free and keep the accuracy over the complete lifetime. The most important advantage is the huge damping. The faceplate is also equipped with a hydrostatic bearing.

Hydrostatic face plate | Image Source: Schiess

The combination of the guides and the bearing take care that the process forces are not transferred to the rest of the machine. No external vibrations disturb the process, thus the process is decoupled. This is extremely important for high precision finishing or the “last cut“ of a workpiece. Additionally, we perform static and dynamic finite element method for all cast parts and modal analysis for the complete machine. The aim is to find resonance vibrations and to shift them in uncritical areas.

These both solutions are parts of the modules to build a machine with high accuracy for years.

But if you want to use hydrostatic you have to solve two challenges:

  • The design:

The design of hydrostatic guides and bearings is a litte bit more difficult than „normal“ ones. But if you master the design it isn’t a problem anymore.

  • The manufacture:

Schiess uses finest milling by machine and scraping by hand. The scraping is an art that requires a great deal of experience and the workers have to fullfil these requirements.

These solutions should be offered only if these two challenges can be mastered. Otherwise, costumers will have a problem a few years down the road.

However, the hydrostatic is not the best solution for every machine‘s requirements. Therefore, Schiess manufacturers also do machining with “normal“ guides and bearings. For the XL machines, the ground machine comprises the components and the customer has the option to change to the hydrostatic version in (almost) all machines.

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